This invention relates to a temperature responsive electric switch provided with a thermal fuse of the non-reset type, operable to open a circuit at a specific temperature. More particularly the invention relates to an improvement of a slidable contact member used in such thermal fuses, wherein the slidable contact member moves when a temperature-sensitive pellet is fused.
It is known to use overheating prevention devices to open a circuit when the temperature of an electric apparatus exceeds a given range, to increase the safety of the apparatus. Conventionally, this kind of overheating preventing device includes two well-known types. One type is the non-reset type employing metals fusible at a specific temperature. Another type is self-resetting and uses bimetal thermocouple means. The non-reset type has the disadvantage that the metal surface degenerates by oxidation or the like with the lapse of time, whereby the response or working temperature is changed, resulting in its unstable function. On the other hand, the bimetal-type is also disadvantageous, in that even when it once functions to open the circuit, a drop in the ambient temperature allows the device to reset and start a current flow. The switch-on and-off operation may thus be repeated unless the power supply is cut off, or the cause of the fault is eliminated. Thus, heat may be gradually stored in the bimetal device also changing the response temperature.
Recently a thermal fuse of the non-reset type employing a temperature-sensitive pellet fusible at a specific temperature has been widely used. This kind of thermal fuse, which houses the temperature-sensitive pellet within a sealed vessel, has the advantage that the fuse is always stable in its function for a long time in the absence of external changes. Such pellet fuses also do not store any heat due to its non-reset characteristic, whereby the desired safety results.
This invention relates to improvements of thermal fuses employing a temperature-sensitive pellet. This type of pellet fuse will now be explained.
A typical conventional thermal pellet fuse of the non-reset type is described, for instance in U.S. Pat. No. 3,519,972. Such pellet fuse comprises a cylindrical housing made of a metal having a good conductivity for electricity and heat. The switching parts include a temperature-sensitive pellet, disc-like metallic plates, and two types of compression spring means arranged within a housing or vessel.
The device further includes a resilient contact member of which the peripheral portion abuts against the inner wall of the metallic housing. These parts are hermetically sealed by means of insulator materials and a lead-in wire passes through the insulator sealing means. The two types of compression springs are formed so that one of the springs has a greater resiliency than the other, while the other spring has a greater restoring force than the former.
In the above structure, the conventional resilient contact member is provided with many circumferential tongues and has, before assembly, the outer diameter D.sub.1 (FIG. 6) sufficiently larger than the inner diameter D.sub.2 of the metallic housing shown in FIG. 3. Such a contact member has a flat base portion and tongues forming contact portions extending outwardly under pressure against the inner wall of the housing to increase the resiliency of the tongues so that the tongues prevent the compression spring having a smaller resiliency from properly expanding or contracting even when the temperature-sensitive pellet is molten, whereby the above mentioned problems occur. Therefore, providing a thermal fuse in which the pellet of meltable material, allows the contact member to separate the contact portion of the lead-in wire quickly, exactly, and to a desired extend, is very desirable.
Besides, this prior art kind of a small-sized thermal fuse is difficult to assemble because it is hard to insert the resilient contact member into the housing. The resilient contact member is especially required to maintain the contact pressure against the inner housing wall after assembly. It is also difficult to achieve an accurate location of the resilient contact member.